We are continuing our study of the structural differences among the amino acids, and complementarily in the transport receptor sites, that divide their transport among the distinct transport mediating systems. Included in the study are the routes by which glycine is taken up both by rat hepatocytes and the HTC hepatoma cell line, the identification of amino acids which allow observation of System ASC without addition of alpha-methylamino acids in various cells, and the comparison of the systems for anionic and cationic amino acid, on the one hand in hepatocytes, and on the other hand in HTC or in cultured skin fibroblasts. Cysteinesulfinic acid has proved useful as a model substrate, along with cysteic acid, for the study of the anionic systems.